This invention is an improvement over the invention disclosed in U.S. Pat. No. 4,427,579--Gaines, Jr. et al. In the Gaines, Jr. et al. invention, amine carbonate or amine carbamate is added to particulate nuclear fuel such that a reaction occurs that forms a water-soluble compound effective as a fugitive binder for the preparation of compact of the nuclear fuel material. The preferred amine compound is ethylene diamine carbamate (abbreviated herein as ECM). The Gaines, Jr. et al. patent is incorporated by reference.
Various materials are used as nuclear fuels for nuclear reactions including ceramic compounds of uranium, plutonium and thorium with particularly preferred compounds being uranium oxide, plutonium oxide, thorium oxide and mixtures thereof. An especially preferred nuclear fuel for use in nuclear reactors is uranium dioxide.
Uranium dioxide is produced commercially as a fine, fairly porous powder, which cannot be used directly as nuclear fuel. It is not a free-flowing powder, but is one which clumps and agglomerates, making it difficult to pack in reactor tubes to the desired density.
The specific composition of a given commercial uranium dioxide powder may also prevent it from being used directly as a nuclear fuel. Uranium dioxide is an exception to the law of definite proportions since "UO.sub.2 " actually denotes a single, stable phase that may vary in composition from UO.sub.1.7 to UO.sub.2.25. Because thermal conductivity decreases with increasing O/U ratios, uranium dioxide having as low as O/U ratio as possible is preferred. However, because uranium dioxide powder oxidizes easily in air and absorbs moisture readily, the O/U ratio of this powder is significantly in excess of that acceptable for fuel.
UO.sub.2 powder synthesized by different methods from uranium hexafluoride gas can exhibit different characteristics, when the UO.sub.2 powder is made into nuclear fuel pellets. As a result, the UO.sub.2 powder may exhibit poor pressibility and require a binding agent in order to form suitable green pellets. The green pellet properties are of particular importance when the powder compacting is conducted in a rotary press. The binder described in the Gaines, Jr. et al. patent is particularly effective. Green pellets prepared therewith exhibit a good combination of tensile strength and plasticity.
In the practice of the Gaines, Jr. et al. invention, a particulate mix of UO.sub.2 requiring the presence of a binder in order to prepare suitable green pellets therefrom is mixed with the amine compound, which is added either as a solid or as a concentrated aqueous solution. The amine compound is substantially uniformly distributed throughout the particulate UO.sub.2 preparatory to the conduct of a reaction to form amine uranyl compounds as the binder material. The reaction temperature to form the amine uranyl compound is maintained at a fairly constant value at atmospheric pressure. The temperature employed can be as low as 68.degree.-72.degree. F. although moderate heating may be desirable. Water, or a source of water in the form of a higher than normal humidity is necessary to carry the reaction to completion. This is conveniently provided in the form of the solvent phase of a solution of the amine carbonate or carbamate added to the particulate fuel. Water may, of course, be added as bulk water, i.e., liquid, or as vapor. The amount of amine carbonate or carbamate (or mixture thereof) added is in the range of from about 0.5% to about 7% by weight of the particulate nuclear fuel. The reacted mass is dried to a water content of about 4000 parts per million before the green pellets are prepared therefrom. The drying is accomplished by passing a stream of dry nitrogen gas through the mixture.
Sintered fuel pellets are required to be slightly porous so that they can accomodate gaseous products generated during the nuclear fission. Sinter densities should typically be in the range of 95-96% of theoretical. In the event that the particulate nuclear fuel is one which sinters to produce a pellet with density that is higher than is desirable, it is necessary to add a pore-forming agent along with the amine compound. As with the binder material, the pore-former must leave the pellet during the sintering operation and leave no residue. Ammonium oxalate is the preferred pore-former.